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A LIVING CELL is like an ancient walled city buzzing with activity. It has
its workshops, storehouses, administrative centre and streets teeming with
traffic, but most importantly of all it is surrounded by a wall that encloses
and defines it. This wall—the plasma membrane—is responsible
for protecting the cell from its hazardous surroundings. Whereas walls were
built to protect cities from armed invaders, the plasma membrane evolved to
protect the cell from substances in its immediate environment such as toxic
acids, alkalis and ions. The evolution of an enclosing membrane was fundamental
to the development of primitive life,…